Continuous granulation of trioxane

ABSTRACT

Method of granulating trioxane in which molten trioxane containing small quantities of water is contacted with a cold surface.

United States Patent Borchmann et al.

(54] CONTINUOUS GRANULATION OF TRIOXANE Appl. No.: 25,561

Foreign Application Priority Data April 5, 1969 Germany ..P l9 l7 647.7

field of Search .264/140, 115, 144

m1 Sept. 5, 1972 Mm Cited UNITED STATES PAIENTS 2.697.249 t2ll9$4Buttes, Jr. et gl. .........264/l l5 FOREIGN PATENTS OR APPLICATIONS731,203 3/ 1966 Canada. .2645

OTHER PUBLICATIONS Buflovak Flakers, Blaw-Knox Co., Catalog 370 pages I,9, 1964.

Primary Examiner-Robert F. White Assistant Examiner-J. R. HallAnomey-Johmton, Root. O'Keeffe, Keil, 'Ilmtpson & Shurtleff ABSTRACTMethod of granulating trioxane in which molten trioxane containing smallquantities of water is contacted with a cold surface.

4Claims,NoDrawlm CONTINUOUS GRANULATION F TRIOXANE It is known thatsubstances which form plastics and soft crystals or which possess a highvapor pressure are difficult to granulate because the products tend tostick to the usual machines or re-agglomerate.

If, for example, the crystallization of trioxane is attempted on a drumflaker in accordance with the proposal made in Dutch Pat. applicationNo. 6,714,150, the result is not usually a flaked product or afree-flowing product of some other shape, but instead there are formedribbons or sheet-like pieces of various thicknesses and widths, whichare difficult to convert to a free-flowing product.

German Pat. specification No. 1,166,212, however, discloses a method ofgranulating trioxane in which molten trioxane is charged into a tankwith walls of polyolefin and allowed to solidify while the tank issuitably agitated.

Although this method of granulating trioxane in tanks with walls ofpolyolefin may produce pure granular trioxane, it is time-consuming andlaborious due to the poor heat transfer through the polyolefin walls,and we have found that good results may only be obtained by charging thecrystallization tank with only relatively small quantities of moltentrioxane.

We have now found that it is possible to produce solid, free-flowingtrioxane of adequate purity for most purposes by a simple andconventional method of crystallization by contacting molten trioxanewith a cold body and scraping the deposited crystalline solid materialfrom the surface of the body, provided that a small quantity of water isadded to the trioxane before, during or after contacting.

The amount of water added may range from, say, to 5,000 ppm and inparticular from 10 to 1,000 ppm based on the weight of the moltenmaterial. Advantageously, from 30 to 150 ppm of water is used. it willbe appreciated that the water may also contain such other materials asare usually added to trioxane, such as antioxidants and stabilizers(phenols, amines, amino alcohols, etc). However, such additives mayalternatively be added to the trioxane beforehand. A suitable cold bodyor crystallizing machine is, in particular, a device known as a drumflaker. A rotary tube with cooled walls and fixed or counter-rotaryinternal scrapers is also very suitable. The usual modifications of suchdevices are also suitable, as for example twin drums. The water isconveniently applied to the surface of the molten trioxane.Alternatively, it may be applied to the metal surface of the cold bodyor added to the trioxane beforehand.

The temperature of the cooling water or that of the surface of the coldbody may be from 5 to 50C, preferably from to 30C. It is beneficial tooperate under an atmosphere of an inert gas, such as nitrogen, sincetrioxane is known to be susceptible to oxygen. The advantage of thepresent method over previously known methods is that large quantities oftrioxane may be granulated in a short time at low expense and withoutappreciable losses. The quantities of water added in the present methodare hardly significant, even for critical applications of trioxane, andsurprisingly, substantially less water is found in the resultingcrystals than was originally added, which leads to the assumption that alarge proportion of the added water evaporates during thecrystallization process.

The method may be conveniently carried out as follows:

Molten trioxane is continuously passed through a heated feed pipe to thetrough of a cooled drum flaker, the drum being simultaneously rotated.Water is dripped continuously on to the surface of the trioxane in thetrough at the desired rate concurrently with the trioxane feed.Alternatively, the water may be previously added to the trioxane, forexample in a stirred vessel, and the water-containing trioxane fed to,say, twin drums. The surface of the drum or drums may be at atemperature of from 5 to 50C, preferably from 15 to 30C. It isbeneficial to fill the drum chamber with an inert gas such as, inparticular, nitrogen. There are practically no trioxane losses. Ifnecessary, small amounts of antioxidants or stabilizers may also beadded to the trioxane to prevent oxidation thereof. Suitable additivesof this kind are phenols, amines, amino alcohols, etc.

Small flake-shaped trioxane crystals, about 0.3 to 5 mm in size, areproduced at the scrapers bearing against the drum, and these crystalsmay be readily filled into sacks, drums or bins, from which they may bereadily removed when required.

EXAMPLE 1 160 kg/h of molten trioxane is pumped through a heatedfeed-pipe to the through of a cooled drum. The latter has a width of1,200 mm and a diameter of 600 mm. When the through is full, the drum isrotated at a speed of 4 rpm. At the same time, 25 g/h of water isdripped on to the surface of the trioxane in the trough via a fine-scaleburette.

The temperature of the cooling water entering the drum is 15C and thatof the expended cooling water emerging from the drum is 18C. The drum isenveloped in nitrogen.

The product scraped off by the scrapers is in the form of flakes havinga mean size of from 2 to 5 mm. The yield is about 159 kg/h of trioxanehaving a water content of about 50 ppm, as determined titrimetrically bythe method of Karl Fischer.

EXAMPLE 2 90 kg of molten trioxane is stirred at a temperature of C and9 g of distilled water is added. The mixture is forced out of the heatedtank by nitrogen pressure and passes through a heated pipe to the nip ofthe twindrum machine. The drums are cooled by water entering at atemperature of 18C. With the nip set at 0.1 mm (under no-loadconditions) and the drums set to rotate at a speed of 6 r.p.m. there areproduced at the scrapers fine needle-shaped trioxane flakes having athickness of from 0.2 to 0.4 mm. The output of the twin drums is 70kg/hr of solid material at a drum width of 500 mm and a diameter of 300mm.

We claim:

1. A method of producing solid, free-flowing trioxane by applying moltentrioxane to the surface of a rotating drum kept at a temperature of from5 to 50C., allowing the trioxane applied to the drum surface tocrystallize, adding a small quantity of at least 10 ppm of water to themolten trioxane before application of the molten trioxane to the drumsurface, and then scraping it off from said surface in the form ofsmall,

tity of water added to the molten trioxane is 10 to 1,000 ppm by weight,based on the trioxane.

4. A method as claimed in claim I wherein the quantity of water added tothe molten trioxane is 30 to ppm by weight, based on the trioxane.

2. A method as claimed in claim 1 wherein the quantity of water added tothe molten trioxane is 10 to 5,000 ppm by weight, based on the trioxane.3. A method as claimed in claim 1 wherein the quanTity of water added tothe molten trioxane is 10 to 1,000 ppm by weight, based on the trioxane.4. A method as claimed in claim 1 wherein the quantity of water added tothe molten trioxane is 30 to 150 ppm by weight, based on the trioxane.